Currently, mammography is an important tool in breast cancer screening and has been shown to reduce mortality by early detection. Despite being a very sensitive technique, mammography's specificity is limited in that only approximately 10-30% of mammographically-directed biopsies are positive. Both PET and single-photon Nuclear Medicine imaging studies using Thallium-201, Technetium-99m Sestamibi, and F-18 Fluorodeoxyglucose have been shown to provide sensitive and specific detection of breast cancers. Nuclear Medicine studies would provide cost-effective methods for Secondary breast cancer screening by limiting the number of negative biopsies. For this Purpose, we propose a dedicated Nuclear Medicine breast imaging device based on NaI(T1) detectors capable of performing both PET and single-photon studies. Since both PET and single-photon radiotracers are likely to play a role in breast cancer detection, a device capable of imaging both tracers is ideally suited to the task. Besides improving the quality of breast imaging over current instrumentation, this device should be sufficiently inexpensive to make Nuclear Medicine breast cancer imaging cost-effective as a secondary Screening method. In this Phase I application, we propose to study the feasibility of this device and begin its overall design.